Water activity in the food industry

Application note: N°F004July 2011Water activity in the food industryFacts & figures:A change in aw of a product canchange the shelf life from acouple of days to a couple ofweeks!Pure distilled water has a wateractivity of exactly 1.Salmonella bacteria can surviveseveral weeks in a dry environment.An example of a sorption isotherm:Discussed in thisedition:Water activity ingeneralWhy the need tomeasure water activity?12Water migration 2Water activity andmicro-organismsWhat solution canRotronic offer?23Rotronic Products 3Customer benefits 4Contact us 5Water activity in generalWater activity:Water activity (a w ) or equilibriumrelative humidity (ERH)measures the water vapourpressure generated by thewater present in a hygroscopicproduct.Water activityis based on ascale from 0 to1.The formulas:a w = p / ps(p is the watervapour pressureabove theproduct surfaceand ps thewater vapourpressure abovethe surface of pure water atthe product temperature)ERH = 100 x a w .Moisture content:Water activity is often confusedwith moisture content.The moisture content of aproduct is usually defined asthe percent weight of watercontent in relation to the dryweight of the sample.Sorption isotherm:At equilibrium, the relationbetween the percentage ofwater and the water activity ofa hygroscopic material can begraphically represented by acurve: the sorption isotherm.For each a w value, the sorptionisotherm shows the correspondingmoisture content at agiven constant temperature.Each product has its ownsorption isotherm.Water migration:The a w of a product will alwaystry to reach equilibrium withthe surrounding atmosphere.Water will migrate from regionswith a high a w to theregions of low a w . Water willmigrate until equilibrium isreached!The effect of water in foodstuffs:Water is also recognised in thefood industry as being criticalfor the stability of most products.a w exerts a decisive influenceon such phenomena aschange in colour, taste andaroma, food poisoning andspoilage (shelf life), loss ofvitamins…Controlling the water activityof a product:The a w in foods can be controlledby using various additives(humectants), by usingsatisfactory packaging materi-Cheese cakeals, by maintaining favourablematuration and storage conditions...Water activity and microorganisms:Water activity also indicatesthe amount of water which isavailable to micro organisms.Each species of micro organism(bacteria, yeast, mould)has a minimum a w value belowwhich, growth is no longerpossible.Chemical stability:SalmonellaWater activity control is alsoan important factor for thechemical stability of foods.Most foodstuffs contain carbohydratesand proteins and aretherefore subject to nonenzymaticbrowning reactions(Maillard reaction). The Maillardreaction gets stronger atincreasing a w values andreached its peak at a w =0.6...0.7 with further increaseof a w this reaction gets rapidlyweaker.

Application note: N°F004July 2011Why the need to measure water activity?Breakfast cerealStay fresh packagingStorage guidelinesWater activity andmicro-organismsWater migrationLate for work one morning,Paul forgets to put the lid backonto his cereals. When cominghome later that evening, Paulfinds that his cereals are allsoft and no longer crunchy…Most breakfast cereals loosetheir crunchiness with aa w >0.4. In this case the relativehumidity of the air inPaul’s kitchen was above 40%rh and the water has migratedfrom the air (with a high levelof relative humidity) to thecereals (with a low a w level).In the food industry, there aretwo main cases where watermigration could cause issues:The first case would be whenAs mentioned previously, a w indicates the amount ofwater in the total water content which is available tomicro-organisms.Each micro-organism has its own minimum a w valuebelow which, growth is no longer possible (growth isno longer possible but this doesn’t mean that themicro-organisms are not present).By measuring the a w of food stuffs it is possible todetermine which micro-organisms will be able todevelop.The US Food and Drug Administration (FDA) hasadopted the concept of a w for establishing moisturelimits beyond which certain types of food are consideredsusceptible to invasion by mould and bacteria.(Please see tables on page 4 for more information).Often, a w is described as the amount of “free” waterin a product and the moisture content, the amount of“bound” water in a product. Even though this is not ascientific description of a w , it is easy to understandthat chemically bound water is not accessible tomicro-organisms, whereas free water is.a wthe finished product containsdifferent components, each ata different a w level.It is important that the a w ismeasured and not the moisturecontent as the moisturecontent will not help with watermigration issues.If we use the example of acheese cake, the cheese hasa a w = 0.95 whereas the biscuitbase has a a w = 0.3. Thewater would migrate from thecheese to the biscuit, leavingyou with a soggy base and adried out cheese.One solution would be to addhumectants (sugar, salt, polymericpolyols…). The humec-Micro organisms generally inhibitedby a w at this point0.950 Pseudomonas, Escherichia, Proteus,Shigella, Klebsiella, Bacillus, Clostridiumperfringens, some yeast.0.910 Salmonella, Vibrio parahaemolyticus,C. botulinum, Serratia, Lactobacillus,Pediococcus, some molds, Rhodotorula,Pichia.0.870 Many yeasts (Candida, Torulopsis,Hansenula), Micrococcus.0.800 Most molds (mycotoxigenic penicillia),Staphylococcus aureus, mostSaccharomyces (baillii) spp., Debaryomyces.0.750 Most halophilic bacteria, mycotoxigenicaspergilla.0.650 Xerophilic molds (aspergillus chevalieri,A. Candidus, Wallemia sebi),Saccharomyces bisporus0.600 Osmophilic yeasts (Saccharomycesrouxii), few molds (Aspergillus echinulatus,Monascus bisporus)tants will lower the a w level butnot the moisture content!Depending on the product it isalso possible to lower the a wlevel by dehydrating and freezing.The second issue would bethe storage of the finishedproduct and the atmosphere inwhich it is stored. Dependingon the a w of the product andthe relative humidity in theatmosphere, water migrationcould occur.The main solutions to avoidwater migration in this case, isto use a non-hygroscopicpackaging or favourable storageconditions.Examples of foods within this range of wateractivity.Highly perishable foods (fresh and cannedfruits, vegetables, meat, fish) and milk; cookedsausages and breads; foods containing up to4oz (w/w) sucrose or 7%NaCl…Some cheese (Cheddar, Swiss, Muenster,Provolone); cured meat (ham); some fruit juiceconcentrates; foods containing 55%(w/w)sucrose or 12%NaCl…Fermented sausage (salami); sponge cakes;dry cheese; margarine; foods containing 65%(w/w) sucrose (saturated) or 15%NaCl…Most fruit juice concentrates; sweetened condensedmilk; chocolate syrup; maple and fruitsyrups; flour; rice; pulses containing 15-17%moisture; fruit cake; country styleham; fondants; high-sugar cakes…Jam, marmalade; marzipan; glazed fruits;some marshmallows…Rolled oats containing ~10% moisture; grainednougats; fudge marshmallows; jelly; molasses;raw cane sugar; some dried fruits; nutsDried fruits containing 15-20% moisture; sometoffees and caramels; honey…0.500 No microbial proliferation Noodles, spaghetti, etc. containing ~12%moisture; spices containing ~10% moisture…0.300 Cookies, crackers, bread crusts, etc. containing3-5% moisture…0.030 Whole milk powder containing 2-3% moisture;dried vegetables containing ~5% moisture;corn flakes containing ~5% moisture; dehydratedsoups; some cookies and crackers…Source: Water activity and Microbial stability, L.R. Beuchat.

Application note: N°F004July 2011What solutions can Rotronic offer?Water activity or equilibriumrelative humidity is usuallydefined as the percent relativehumidity generated inequilibrium with the productsample in a closed system atconstant temperature.ERH = 100 x a wTherefore, a w can be measuredwith a relative humiditysensor provided that theconditions specified in theabove definition are fulfilled.Practical conditions formeasuring water activity:Leak proof measurementchamber: closed system.Volume ratio: air/product.The volume of air must bekept to a minimum: a smallair volume reaches equilibriumwith the sample fasterthan a large air volumeTemperature homogeneity:any temperature differencebetween the sensor, thechamber and the sample willresult in significant errors.The higher the a w value thegreater the error will be (a0.8a w at 25°C with a 1°Ctemperature difference canresult in an error of 0.05a w ).Constant temperature needsto be kept.Equilibrium time: in order toread the correct water activityvalue, equilibrium needsto be reached! The higherthe a w the longer it takes!Calibration of the sensor:using traceable humiditystandards.Rotronic offers a complete a w measurementproduct range!HP23-AW-A with the HC2-AW andHC2-P05Rotronic products:Humidity and temperatureprobes:• HC2-AW5...50°C,0…1a w,Large thermal mass,±0.008a w and ±0.1K…• HC2-AW-USBSame as HC2-AW but withdirect connection to PC.• HC2-P05Insertion probe,-40...85°C,0…1a w,Ø5mm x 200mm,±0.015a w and ±0.3K…• HC2-HP28• HC2-HP50Insertion probe,40...85°C,0…1a w,Ø10mm x 280 or 500mm,±0.008a w and ±0.1K…Laboratory display units:• HygroLab C14 probe connections,Data logging,Display,Ethernet & USB connection,AwE and AwQuick,Buzzer...• HP23-AW-AHand held device,2 probe connections,Data logging,Display,AwE & AwQuickBuzzer…Accessories:• Sample holdersWP-14-S, 14mm depth,WP-40, 40mm depth.• Disposable samplecontainersPS-14, containers for WP-14-S,PS-40, containers for WP-40• Clamp sealing mechanism:AW-KHS, sealing clamp• SCS humidity standardsEAxx-SCS, unsaturated saltsolutions,Different humidity values available:0, 5, 10, 11, 20, 35, 50,60, 65, 75, 80 and 95%...Water activity sets:Various sets are available, pleasecontact us for more details.HygroLab C1WP-14-SAW1-SET-40AW-KHS sealing clampHC2-P05HC2-HP28 or HC2-HP50WP-14-S

Application note: N°F004July 2011Customer benefits:How Rotronic measures wateractivity:With the Rotronic product range,there are two different ways to measurewater activity: AwE andAwQuick.AwE mode: the natural equilibrationof the product is measured and themeasurement process is automaticallystopped once equilibrium isreached. With most products, naturalequilibrium requires from 45 to 90minutes.AwQuick mode: this mode reducesthe time required to measure wateractivity to a few minutes, usually withalmost the same accuracy as theAwE mode.Water activity measurement reports:Combined with the Rotronic HW4software, it is possible to automaticallygenerate a report as soon asthe measurement is finished.Rotronic water activity sets:Rotronic offers different sets, offeringa complete solution for everyoneneeding to measure water activity.The sets often contain, a displayunits, a a w measurement device, asample holder and disposable containersas well as Rotronic humiditystandards for the calibration of themeasurement device.Accuracy and long term stability:Choosing Rotronic gives you thebest accuracy on the market.This helps carry out fast and effectivewater activity measurements onall foodstuffs.With a long term stability of under0.001a w , the measurement deviceswill not need much taking care of!This being said, we would recommendfrequent spot checks inbetweencalibrations.Calibration and adjustment:Calibration and adjustment is veryeasy with the Rotronic productrange. As all of the communication isdigital, the whole calibration procedurecan be done via a PC, or directlyfrom the display unit (HP23-AW-A or the HygroLab C1) with thehelp of the Rotronic humidity standards.Rotronic can also offer a factorycalibration (certified or not).FDA Food code 2005: Potentially hazardous foodRotronic HW4 Software

Application note: N°F004Contact us:July 2011Rotronic is represented in over 40 countries around the world. An up to date list of all our partners is available on our website:www.rotronic.comSWITZERLANDROTRONIC AGGrindelstrasse 6,CH-8303 BassersdorfPhone: +41 44 838 11 44Fax: +41 44 837 00 73www.rotronic.comFRANCEROTRONIC Sarl56, Bld. De Courcerin,F-77183 Croissy-Beaubourg.Phone: +33 1 60 95 07 10Fax: +33 1 60 17 12 56www.rotronic.frSINGAPOREROTRONIC South East Asia Pte Ltd16 Kallang Place #07-04Singapore 339156Phone: +65 6294 6065Fax: +65 6294 6096www.rotronic.com.sgGERMANYROTRONIC Messgeräte GmbHEinsteinstrasse 17-23DE-76275 EttlingenPhone: +49 7243 383 250Fax: +49 7243 383 260www.rotronic.deUKROTRONIC Instruments UK Ltd.Crompton Fields, Crompton WayCrawley, West Sussex, RH10 9EEPhone: +44 1293 57 10 00Fax: +44 1293 57 10 08www.rotronic.co.ukITALYROTRONIC Italia srlVia Repubblica di San Marino, 1I-20157 Milano (MI)Phone: +39 02 39 00 71 90Fax: +39 02 33 27 62 99www.rotronic.itUSAROTRONIC Instrument Corp.Suite 150, 135 Engineers Road, Hauppauge,NY 11788Phone: +1 631 427 38 98Fax: +1 631 427 39 02www.rotronic-usa.comCHINAROTRONIC Shanghai Rep. Office2B, Zao Fong Universe Building, No. 1800ZhingShan West Road, Shanghai 200233ChinaPhone: +86 21 644 03 55Fax: +86 21 644 03 77www.rotronic.cn